JP4154885B2 - Welded joint made of Ni-base heat-resistant alloy - Google Patents

Welded joint made of Ni-base heat-resistant alloy Download PDF

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JP4154885B2
JP4154885B2 JP2001348898A JP2001348898A JP4154885B2 JP 4154885 B2 JP4154885 B2 JP 4154885B2 JP 2001348898 A JP2001348898 A JP 2001348898A JP 2001348898 A JP2001348898 A JP 2001348898A JP 4154885 B2 JP4154885 B2 JP 4154885B2
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weld
weld metal
metal
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JP2002235136A (en
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和博 小川
浩一 岡田
佳孝 西山
和潔 來村
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Sumitomo Metal Industries Ltd
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Sumitomo Metal Industries Ltd
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Description

【0001】
【発明の属する技術分野】
本発明は、熱間加工性、溶接性および耐浸炭性に優れた高温強度の高いNi基耐熱合金からなる溶接継手、さらにはその溶接継手を有するエチレンプラント用分解炉および改質炉に使用される溶接構造を前提とする管に関する。エチレンプラント用分解炉および改質炉とは、ナフサ、プロパン、エタン、ガスオイル等の炭化水素原料を800℃以上の高温で分解または改質し、エチレン、プロピレン等の石油化学基礎製品を製造する炉である。
【0002】
【従来の技術】
エチレンプラント用分解炉および改質炉の使用温度は、エチレン等の収率向上の観点から高温化の傾向が強くなってきている。
【0003】
このような分解炉および改質炉の管用材料としては、内面が浸炭雰囲気に曝されるため、高温強度と耐浸炭性に優れた耐熱性が要求される。また、一方では、操業中にコーキングと称される管の内表面に炭素が析出する現象が現れ、その析出量の増加に伴い管内圧力の上昇や加熱効率の低下などの操業上の弊害が生じる。
【0004】
したがって、実操業においては、定期的に空気や水蒸気で析出した炭素を除去する、いわゆるデコーキング作業がおこなわれているが、その間の操業停止や作業の工数などが大きな問題になる。このようなコーキングとそれに伴う諸問題は、管のサイズが収率向上に有利な小径管になるほど深刻になる。
【0005】
コーキング防止を目的とした従来技術としては、例えば、特開平2−8336号公報に示されるように、合金中に28質量%以上のCrを含有させて合金表面に強固で安定なCr皮膜を形成させ、炭素析出を促進する触媒元素であるFeおよびNiの表面への露出を防止してコーキングを抑制するようにした技術がある。
【0006】
一方、耐浸炭性の向上のためには、例えば、特開昭57−23050号公報に示されるように、合金中のSi含有量を高めるのが有効なことが知られている。
【0007】
しかしながら、これらの従来技術には次のような問題点がある。
【0008】
コーキング防止の観点から、特開平2−8336号公報に提案されるような高Cr合金を高温強度部材として適用する場合には、合金中のNi量を高めて金属組織をオーステナイトにする必要がある。しかし、単にオーステナイト組織にするだけでは、高温強度が従来合金に比べて低いので単独では高温強度部材として適用することは難しい。なお、特開平2−8336号公報には、他の高温強度部材と組み合わせて二重管とし使用することが示されているが、二重管は製造コストや信頼性の点で問題が多い。
【0009】
また、特開昭57−23050号公報に示されるように、合金中のSi含有量を高めた場合には、溶接割れ感受性が高くなり、溶接構造物としての実用に耐えないという問題がある。
【0010】
これに対して、特開平4−358037号公報、同5−239577号公報、同5−33092号公報および同6−207235号公報に示されるように、合金中のAl量を高めてメタル表面に強固で緻密なAl皮膜を生成させるようにした合金は、従来の合金に比較して耐浸炭性および耐コーキング性が著しく向上する。また、このような高Al合金では、Ni量を高めることにより高温での使用中にγ’相がマトリックス中に微細に析出し、クリープ破断強度も大幅に向上する。すなわち、これらの公報に示される合金は、高温での耐浸炭性、耐コーキング性に優れ、しかもクリープ強度が高いことが特徴で、エチレンプラント用分解炉および改質炉管として好適である。
【0011】
しかし、上記の公報に示される合金は、溶接性、特に耐溶接割れ性に対する成分設計面での配慮が十分になされていないだけでなく、溶接継手を構成する溶接金属自体の成分設計についても十分な配慮がなされていなかった。Al量の高いNi基合金では、溶接時の溶接熱影響部(Heat Affected Zone;以下、HAZともいう)および溶接金属で割れを生じやすいのみならず、溶接金属では母材に比べて高温クリープ強度が低くなりやすい。
【0012】
溶接金属は、母材のように熱間加工、熱処理を受けた状態ではなく、凝固組織のままで使用されるため、高温クリープ強度が低くなりやすい。したがって、実用に有益な材料とするには、母材および溶接金属の成分設計において溶接時の割れ感受性の低減と溶接継手のクリープ強度の低下防止を織り込んでおくことが重要な課題となる。
【0013】
【発明が解決しようとする課題】
本発明の目的は、エチレンプラント用分解炉および改質炉の管がおかれる環境、すなわち浸炭、酸化および温度変動が繰り返される環境下において優れた耐浸炭性と耐コーキング性を有し、かつ優れた溶接性と高温強度を有するNi基耐熱合金からなる溶接継手を提供することにある。
【0014】
【課題を解決するための手段】
本発明の要旨は、下記(1)のNi基耐熱合金からなる溶接継手にある。以下、成分の含有量に関する%は質量%を意味する。
【0016】
(1)母材および溶接金属がいずれも、C:0.1%以下、Si:2%以下、Mn:2%以下、P:0.025%以下、S:0.005%以下、N:0.04%以下、Cr:10〜30%、Al:2.1〜4.5%未満、並びにMo:2.5〜15%もしくはW:2.5〜9%またはMoとWの両方を合計で2.5〜15%含み、さらにTi:0〜3%、Nb:0〜1%、Ta:0〜2%、Zr:0〜0.2%、Hf:0〜0.8%、B:0〜0.03%、Mg:0〜0.01%、Ca:0〜0.01%、Fe:0〜10%、La:0〜0.1%、Ce:0〜0.1%、Nd:0〜0.1%、Y:0〜0.1%、Cu:0〜5%、Co:0〜10%を含み、残部がNi および不純物よりなり、かつ下記の(1) 式を満足するとともに、下記の(2)式または(3)式で求められる溶接金属のST値が母材のST値よりも大きく、その差が3以上であるNi基耐熱合金からなる溶接継手。
【0017】
(104Si+1980P+1980S+9Al+15Ti+11Nb+1.8W+11600B)
≦{1.1(240−20000S−1900P−30Al−10Ti−9W+17000B)}・・・(1)
Ti≦4Cの場合ST=Mo+1.5W+100Ti・・・(2)
Ti>4Cの場合ST=Mo+1.5W+400C・・・(3)
ここで、(1)〜(3)式中の元素記号は、母材または溶接金属中に含まれる各元素の含有率(質量%)を意味する。
【0018】
上記の成分の中で、Tiは0.005〜1.0%であることが望ましい。
【0019】
本発明のNi基合金からなる溶接継手は、特にエチレン分解炉および改質炉の管として用いるのに好適である。
【0020】
【発明の実施の形態】
本発明者等は、前記の目的を達成するため、Cr含有量が10〜30%、Al含有量が2.1から4.5%未満で、かつ2.5〜15%のMoおよび2.5〜9%のWのうちの一方または両方(但し、両方を含む場合は合計で2.5〜15%)を含むNi基合金を対象に、実験、研究を重ねた結果、次のことを知見し、本発明を完成させた。
【0021】
HAZでの割れは、溶接熱サイクルを受けた際に溶接金属との境界に近い高温に加熱された母材の粒界が、一部溶融した母材部位を起点として割れ、この割れが粒界脆化を生じたより低温側のHAZに伝播することにより生じる。すなわち、HAZでの溶接割れは、溶接熱応力が上記のようにして低下した粒界の固着力を上回ることにより発生する。
【0022】
そこで、粒界の部分溶融とそれよりも低温側で生じる粒界脆化に及ぼす合金元素の影響を定量的に評価し、抵抗力を定量評価することを試みた。
【0023】
その結果、溶接熱応力Rのもとで粒界溶融量BIの部材から亀裂が生じた際のエネルギ−開放率Aが、下記の(4) 式を満足すれば、たとえ粒界が部分的に溶融しても割れは生じないことがわかった。
【0024】
(A=C×R ×BI)≦DI ・・・・ (4)
ここで、Cは定数であり、DIは粒界固着力を表す。BIとDIは合金の化学組成に依存する。即ち、BIは粒界溶融を生じさせやすい元素(例えば、Si、P、Al、Ti、Nb、W、B)の含有率が多いほど大きくなり、DIは粒界脆化を生じさせやすい元素(例えば、S、P、Al、Ti、W)の含有率が多いほど小さくなる。従って、粒界脆化を生じさせやすい元素の含有量が増えると、(4) 式を満足させることが難しくなる。
【0025】
なお、粒界溶融量BIは、理論的には、各元素iに対し、平衡状態図における元素1%当たりの液相線温度の低下度をmi、固液相分配係数をki、粒界偏析量をbi、各元素の含有率をXiとした場合、下記の(5) 式により求められるが、実験の結果、下記の(6) 式で求められる値とすればよいことがわかった。
【0026】
BI=Σ{mi/(bi−1)/ki}×Xi ・・・・ (5)
BI=104Si+1980P+1980S+9Al+15Ti+11Nb+1.8W+11600B・・(6)
また、粒界固着力DIは、粒界でのNi原子の結合力に及ぼす各元素の影響を高温での破壊テストにより定量する実験の結果、下記の(7) 式で求められる値とすればよいことがわかった。
【0027】
DI=240−20000S−1900P−30Al−10Ti−9W+17000B ・・・ (7)
一方、溶接熱応力Rは、厳密には溶接条件や溶接継手の形状寸法、特に板厚や肉厚の影響を受けるが、TIG溶接を主とするNi基耐熱合金では、入熱量で高々20 kJ/cm程度であり、その程度の入熱量の影響は大きくない。
【0028】
しかし、板厚の溶接熱応力に及ぼす影響は大きく、溶接熱応力Rは、板厚10mmまでは板厚の増加とともに急激に増大する。このため、溶接構造物として考えられる厚さの上限に近い板厚25mmにて、最も厳しい完全拘束条件下で模擬溶接実験を行って割れ発生の有無を調査した。
【0029】
その結果、上記(6) 式で求められるBI値が(7) 式で求められるDI値の1.1倍以下、つまり前述した(1)式を満足するように成分調整すれば、溶接時にHAZで割れが生じないことがわかった。
【0030】
次に、前述した(1)式を満足するように成分調整された母材を前提に、溶接割れ感受性が低く、かつ母材と同等のクリープ強度を有する溶接継手が得られる溶接金属の開発に努めた。
【0031】
溶接金属のクリープ強度が母材に比べて低下するのは、母材ではマトリックス中に固溶して強化に有効に寄与していたMoおよび/またはWが溶接金属では凝固偏析により母材ほど有効には寄与しないことによる。したがって、溶接金属には母材よりも多くのMoおよび/またはWを含有させる必要がある。
【0032】
しかし、母材が十分に高い含有量のMoおよび/またはWを有している場合、溶接金属にさらに多くのMoおよび/またはWを含有させることは困難である。溶接金属に多量のMoおよび/またはWを含有させるためには、溶接材料のMoおよび/またはWの含有量を高めなければならず、そうすると、溶接材料に加工する際の熱間加工性が低下するからである。
【0033】
そこで、溶接金属に母材よりも多量のMoおよび/またはWを含有させなくてもそのクリープ強度が母材よりも低くならない条件を探求した。その結果、溶接金属の粒界に適量のTiCを分散させるのが有効なことがわかった。具体的に説明すると、母材または溶接金属中に含まれるTi含有量に応じて前述した(2)式または(3)式で求められるST値が、母材よりも溶接金属の方が大きく、その差が3以上になるように母材と溶接金属のMo、W、CおよびTi含有量を調整すれば、凝固組織である溶接金属のクリープ強度が母材のクリープ強度とほぼ同じになることがわかった。
【0034】
多層溶接時には溶接金属は、次層の溶接によってHAZと同じ熱サイクルを受け、母材のHAZ割れと同様の割れを生じる。しかし、溶接金属が前述した(1)式を満足する場合には、母材と同様に、溶接割れは生じないことがわかった。
【0035】
なお、本発明の溶接継手の母材合金の基本組成に関しては、本発明者等は、高温での耐浸炭性、耐コーキング性を低下させることなく、前述の溶接性に加え実用量産合金として必要不可欠な熱間加工性を満足させる観点から、種々の化学組成の合金を溶製して実験、検討を重ねた結果、以下の知見を得た。
【0036】
(a) Alを1%以上含有する合金においては、Al系窒化物を形成しやすく、この窒化物系析出物を起点にAlOを主体とするAl系酸化物皮膜の保護性が失われる。
【0037】
(b) しかし、Nの固溶度を高める効果のあるCrを10%以上含有させる一方、Nを低減すれば、Nは十分に固溶してAlNが合金表面に析出しない。従って、Alが1%以上であっても、合金表面のアルミナ主体の酸化皮膜の保護性が損なわれず、良好な耐浸炭性と耐コーキング性が確保でき、しかも高温強度が向上する。
【0038】
(c) Al含有量を4.5%未満に抑えると、熱間加工性と溶接性は向上するが、その熱間加工性は、一般のFe−Cr−Ni系やNi−Cr系合金と比較すると、量産化を考慮した場合十分とは言えない。すなわち、熱間加工時にNi−Al系金属間化合物が析出し、結晶粒内が著しく強化されるために相対的に粒界が弱くなる。粒内が強化されると加工の際に大きな力を加えなければならず、そうすると相対的に弱い粒界が破壊しやすく、熱間加工性が低下する。そこで、熱間加工性を改善するためには、粒内の強化と同等に粒界も強化する必要がある。
【0039】
(d) 一方、Alを多く含有するNi基合金は、粒界そのものが弱化している。この弱化の主要因のひとつがSである。従って、粒界の弱化を防ぐにはSを0.005%以下に制限することが極めて重要であり、0.003%以下に制限すればなお一層の改善効果が期待できる。
【0040】
(e) さらにNを可能な限り低くすることが重要である。多量のAlを含有するNi基合金では、前述したように鋼中のNがAl系窒化物を形成しやすく、この窒化物系析出物が熱間加工性を著しく低下させるからである。
【0041】
(f)B、ZrおよびHfは、粒界での原子の結合力を高めるため、粒界の強化に効果を発揮するので、熱間加工性の低下防止にはこれらの元素の1種以上を含有させるのがよい。
【0042】
以下、本発明のNi基耐熱合金からなる溶接継手の母材と溶接金属の化学組成を上記のように定めた理由について詳細に説明する。以下の各元素の説明等は、特に断らない限り、Ni基耐熱合金からなる溶接継手を構成する母材と溶接金属に共通である。
【0043】
本発明のNi基耐熱合金からなる溶接継手を構成する母材と溶接金属は、いずれも、下記の(1) 式を満足する必要がある。
【0044】
(104Si+1980P+1980S+9Al+15Ti+11Nb+1.8W+11600B)
≦{1.1(240−20000S−1900P−30Al−10Ti−9W+17000B)}・・・(1)
ここで、(1)式中の元素記号は、母材の合金または溶接金属中に含まれる各元素の含有率(質量%)を意味する。
【0045】
上記の(1)式を満たすことは、溶接割れ防止のために必須の条件である。(1)式の意味は次のとおりである。前述したように、溶接により融点直下となった領域での粒界の部分的な溶融量に依存して生じる局部的な破壊応力が、隣接した粒界の脆化域での破壊抵抗を上回らない範囲内に成分の組み合わせを選択することを意味し、(1)式を満足する場合に限って溶接割れが発生するのを防ぐことが可能となる。
【0046】
本発明の溶接継手は、次の条件を満足しなければならない。
【0047】
継手を構成する母材と溶接金属のそれぞれのST値を下記の(2) 式または(3)式で求めたとき、溶接金属のST値が母材のST値よりも大きく、その差が3以上であることである。この場合に限って固溶強化とTiC分散による強化の相乗効果により、凝固組織である溶接金属のクリープ強度が母材のクリープ強度と同等になってバランスする。
【0048】
Ti≦4Cの場合ST=Mo+1.5W+100Ti・・・(2)
Ti>4Cの場合ST=Mo+1.5W+400C・・・(3)
ただし、上記の2つの条件は、以下に示す合金成分の範囲内において満足する必要がある。このことは、後述する実施例の結果からも明らかである。
【0049】
C:0.1%以下
Cは、炭化物を形成して耐熱合金として必要な引張強さやクリープ破断強度を向上させるためには有効な元素であるから、0.01%以上含有されることが望ましい。しかし、その含有量が0.1%を超えると合金の延性および靭性の低下が大きくなるばかりでなく、Alを多く含むNi基合金においてはアルミナ皮膜形成を阻害する。このため、C含有量は0.1%以下とした。好ましい上限は0.09%、より好ましい上限は0.07%である。
【0050】
Si:2%以下
Siは、脱酸剤として添加される元素であり、耐酸化性や耐浸炭性改善にも寄与する元素であるが、Alを多く含有するNi基合金においては耐酸化性や耐浸炭性の改善効果が比較的小さい反面、熱間加工性や溶接性を低下させる作用が強い。このため、製造上、特に熱間加工性が重視される場合には低い方がよいが、耐酸化性や耐浸炭性の改善作用を得る必要がある場合もあることを考慮して2%以下とする。好ましい上限は1.5%、より好まし上限は1%である。なお、耐酸化性や耐浸炭性の改善作用は、0.2%以上で顕著になる。
【0051】
Mn:2%以下
Mnは、脱酸剤として添加される元素であるが、耐コーキング性の劣化要因となるスピネル型酸化物の皮膜形成を促進することから2%以下に抑える必要がある。好ましい上限は1.5%、より好ましい上限は1%である。Mn含有量は不純物レベルでもよいが、脱酸効果を確実にするためには、0.1%以上含有させるのが望ましい。
【0052】
S:0.005%以下
Sは、粒界に偏析して結晶粒の結合力を弱め、溶接性を劣化させる極めて有害な元素であり、上限値の規制が極めて重要である。特に、Alを多く含むNi基合金では粒界強化が重要となるため、S含有量は極力低減するのが好ましい。また、溶接性を改善するためには少なくとも0.005%以下とする必要があることからその上限を0.005%とした。
【0053】
P:0.025%以下
Pは、粒界に偏析して結晶粒の結合力を弱めるとともに、粒界の融点を下げ、高温HAZ(溶融境界に接する母材部分)での粒界の部分溶融を促進して溶接割れを生じさせる有害な元素であり、その含有量は低ければ低いほど望ましいが、0.025%までであれば特に問題ないことから、その上限を0.025%とした。
【0054】
N:0.04%以下
Nは、一般の耐熱鋼においては固溶強化により高温での強度を高めるのに有効で積極的に用いられているが、Alを多く含むNi基合金では、AlN等の窒化物として析出するために固溶強化が期待できないばかりか熱間加工性、溶接性を著しく阻害する。さらに、窒化物を起点として保護性皮膜を破壊し耐浸炭性を低下させる。したがって、N含有量は低ければ低いほどよいが、0.04%までであれば特に問題ないことと、過度な低減はコスト上昇と歩留まり低下を招くことからその上限を0.04%とした。なお、好ましい上限は0.03%、より好ましい上限は0.02%である。
【0055】
Cr:10〜30%
Crは、耐酸化性や耐コーキング性の改善に有効な元素であり、アルミナ皮膜の生成初期においてアルミナ皮膜を均一に生成させる作用がある。また、炭化物を形成しクリープ破断強度の向上にも寄与する。さらに、本発明で規定する成分系においてはCrは熱間加工性の向上にも寄与する。これらの効果を得るためには最低でも10%以上が必要がある。一方、Crを過剰に含有させると靭性、加工性といった機械的性質を阻害することになる。このため、Cr含有量は10〜30%とした。好ましい範囲は12〜25%、より好ましい範囲は12〜23%である。
【0056】
Al:2.1%から4.5%未満
Alは、耐浸炭性および耐コーキング性の向上、さらには高温強度の向上に極めて有効な元素であるが、その効果を得るにはコランダム型のアルミナ酸化皮膜を均一に生成させる必要がある。また、Alはγ’相[Ni (Al、Ti)の金属間化合物]を形成して析出強化作用を発揮する。これらの効果を得るためには最低でも2.1%以上が必要である。一方、4.5%以上になると溶接割れ感受性が極端に増大する。したがって、Al含有量は2.1から4.5%未満とした。好ましい範囲は2.1〜4%、より好ましい範囲は2.1〜3.5%である。
【0057】
Mo:2.5〜15%、W:2.5〜9%、但し両者を含む場合は合計で2.5〜15%
これらの元素は、いずれも、主として固溶強化元素として有効であり、基地のオーステナイト相を強化することによりクリープ破断強度を上昇させる。その効果を得るにはそれぞれ2.5%以上、2種の合計含有量でも2.5%以上が必要である。しかし、過剰に含有させると靭性低下の要因となる金属間化合物が析出するだけでなく、耐浸炭性や耐コーキング性も劣化する。上限はMoとWの合計で15%以下に抑えるべきである。ただし、Moに比べてWは金属間化合物析出による熱間加工性および溶接性低下が大きいため、MoよりもWの上限を低く制限する必要がある。このため、これら元素の含有量は、Moで15%以下、Wで9%以下の範囲内で、合計でも15%以下とした。合計量の好ましい範囲は4〜13%、より好ましい範囲は6〜13%である。
【0058】
B:
Bは添加しなくてもよい。添加すれば、粒界を強化する作用があり、溶接割れ感受性の低減に寄与する。このため、この効果を得たい場合に添加することができ、その効果は含有量0.001%以上で顕著になる。しかし、Bの含有量が0.03%を超えると、かえって溶接割れ感受性が高くなる。したがって、添加する場合のB含有量は0.001〜0.03%とするのがよい。
【0059】
Ti:
Tiは添加しなくてもよい。添加すれば、γ’相の析出を促進してクリープ破断強度の向上に寄与する他、TiCとして析出して粒界を強化し、溶接金属のクリープ破断強度の向上にも寄与する。このため、これらの効果を得たい場合に添加することができ、その効果は含有量0.005%以上で顕著になる。しかし、3%を超えて含有させると、γ’相が過剰に析出して溶接性が著しく劣化する。したがって、添加する場合のTi含有量は0.005〜3%とするのがよい。なお、好ましい上限は1%である。
【0060】
Zr、Hf:
これらの元素は添加しなくてもよい。添加すれば、いずれの元素も、粒界に偏析して粒界すべりを抑えることによってクリープ強度の向上に寄与する。このため、この効果を得たい場合には1種以上を添加することができ、その効果は、いずれの元素も、0.01%以上で顕著になる。しかし、Zrが0.2%を超える場合、Hfが0.8%を超える場合、いずれも、かえってクリープ破断強度が低下する。したがって、添加する場合のZr含有量は0.01〜0.2%、Hf含有量は0.01〜0.8%とするのがよい。
【0061】
Mg、Ca:
これらの元素は添加しなくてもよい。添加すれば、いずれの元素も、主として熱間加工性に有害なSを硫化物として固定して粒界強度を高め、熱間加工性の改善に寄与する。このため、この効果を得たい場合には1種以上を添加することができ、その効果は、いずれの元素も含有量0.0005%以上で顕著になるが、0.01%を超えると、固溶状態で合金中に存在し、逆に熱間加工性および溶接性を低下させる。したがって、添加する場合これら元素の含有量は、いずれも0.0005〜0.01%とするのがよい。なお、これらの元素を添加する際のMgとCaの含有量は、式「(1.178Mg+Ca)/S」で求まる値が0.5〜3の範囲内に入るように含有させるのが望ましく、この場合には熱間加工性の改善効果が一段と向上する。
【0062】
Fe:
Feは添加しなくてもよい。添加すれば、クリープ延性を改善し、クリープ破断強度の向上に寄与する他、熱間加工性や常温加工性の改善にも寄与する。このため、この効果を得たい場合には添加することができ、その効果は含有量0.1%以上で顕著になる。しかし、10%を超えると、逆にクリープ破断強度、熱間加工性とも低下する。したがって、添加する場合のFe含有量は0.1〜10%とするのがよい。
【0063】
Nb Ta
これらの元素は添加しなくてもよい。添加すれば、いずれの元素も、オーステナイト相に固溶して基地を強化する他、炭化物を形成してクリープ破断強度の向上に寄与する。このため、この効果を得たい場合には1種以上を添加することができ、その効果は、いずれの元素も含有量0.01%以上で顕著になるが、Nb 1%を超えると、また、Taは2%を超えると、靭性低下を招く。したがって、添加する場合のNb 含有量は0.01〜1%、Taの含有量は0.01〜2%とするのがよい。なお、Nb 好ましい上限は0.8%、Taの好ましい上限は1.8%である
【0064】
La、Ce、Nd、Y:
これらの元素は添加しなくてもよい。添加すれば、いずれの元素も、主として熱サイクル条件下でのアルミナ皮膜の剥離を防止し、温度が変動する環境下での使用においても耐浸炭性および耐コーキング性を向上させる効果がある。したがって、その効果を得たい場合には1種以上を添加することができ、その効果は、いずれの元素も、含有量0.002%以上で顕著になる。しかし、いずれの元素も、0.1%を超えると、アルミナ皮膜の剥離防止効果が飽和するばかりでなく、加工性が悪化する。したがって、添加する場合のこれら元素の含有量は、いずれの元素も、0.002〜0.1%とするのがよい。
【0065】
Cu、Co:
これらの元素は添加しなくてもよい。添加すれば、いずれの元素も、主としてオーステナイト相の安定化に寄与する他、Coは固溶強化によりクリープ強度の向上にも寄与する。このため、これらの効果を得たい場合には1種以上を添加することができ、その効果は、いずれの元素も、含有量0.01%以上で顕著になる。しかし、Cuは5%、Coは10%をそれぞれ超えると、靭性および加工性が損なわれる。したがって、添加する場合のCu含有量は0.01〜5%、Co含有量は0.01〜10%とするのがよい。なお、Cu含有量の好ましい上限は3%、より好ましい上限は1.5%であり、Co含有量の好ましい上限は8%、より好ましい上限は5%である。
【0066】
本発明の溶接継手の母材となるNi基耐熱合金は、通常の溶解および精錬工程で溶製した後、鋳造することにより得られ、鋳造のままでも用いることができる。通常、鋳造後に鍛造、熱間加工、冷間加工等の各加工工程を経て用いる。なお、粉末冶金法で製造してもよい。熱処理は組織の均一化を促進し、本発明に係る溶接継手の性能向上に寄与する。熱処理としては、1100〜1300℃での均一化処理が好ましいが、鋳造あるいは加工のままでの使用も可能である。
【0067】
また、本発明の溶接継手の母材となる Ni 基耐熱合金は線材に加工して共金系の溶接材料として使用することでき、これを用いてTIG溶接等により溶接継手が得られる。その際の溶接金属は、使用性能、溶接割れ防止の観点から前記の組成とする。その組成範囲内で前記の(2)式または(3)式で求められる溶接金属のST値が母材のST値よりも大きく、その差が3以上になるようにする必要がある。それによって、母材と同等のクリープ強度を有する溶接金属が得られる。
【0068】
溶接金属は溶接材料と母材の一部とが溶融混合して形成される。従って、溶接金属の組成は、母材組成と希釈率を考慮して溶接材料の組成を選ぶことによって調整することができる。
【0069】
【実施例】
表1に示す化学組成を有する20種類の母材と、表2に示す化学組成を有する9種類の溶接材料を準備した。なお、母材および溶接材料は、いずれも、容量50kgの高周波真空溶解炉を用いて溶製し、得られたインゴットを熱間鍛造して板厚25mmの板材とし、1250℃で固溶化熱処理を施したものを母材とした。また、上記のインゴットを熱間鍛造、熱間圧延、冷間引抜きの工程を経て外径2mmの線材に加工して溶接材料とした。
【0070】
【表1】
【0071】
【表2】
【0072】
準備した母材および溶接材料を用いて、下記要領による溶接継手の作製試験を行った。
【0073】
各母材から、幅100mm、長さ200mmで、長辺の一方に半角20゜のV開先加工を施した2枚の試験片を採取した。この2枚の試験片は、V開先加工を施した長辺同士を突き合わせ、厚さ50mm、幅150mm、長さ250mmの鋼板上に載置してその4辺全周を被覆アーク溶接(3パス)して完全に拘束した。次いで、突き合わせたV開先部分をTIG溶接法にて多層溶接した。溶接条件は溶接電流130A、溶接電圧12V、溶接速度15cm/minとした。準備した母材と溶接材料とを種々組合せ、表3に示す化学組成の溶接金属をもつ23種類の溶接継手を作製した。
【0074】
溶接性(耐溶接割れ性)の評価は、得られた各溶接継手から、長手方向が溶接線と直交する方向で、長手方向の中央に溶接金属の幅方向中央が位置する厚さ15mm、幅10mm、長さ200mmの側曲げ試験片を各5個採取し、曲げ半径20mmで180度曲げを行って曲げ部の表面を50倍の拡大視野にて検鏡し、HAZおよび溶接金属での割れの発生の有無を調べ、試験片5個ともに割れの発生が認められなかったものを耐溶接割れ性が良好「○」、試験片1個にでも割れの発生が認められたものを耐溶接割れ性が不良「×」として評価した。
【0075】
また、割れの発生が認められなかった溶接継手については、長手方向が溶接線と直交する方向で、長手方向の中央部に外径6mm、長さ30mmの平行部を有し、この平行部の中央に溶接金属が位置するクリープ破断試験片を採取し、温度1150℃、負荷応力7MPaの条件によるクリープ破断試験を行って溶接金属の破断時間を調べた。
【0076】
溶接金属のクリープ強度評価は、母材から採取して行った上記と同じ条件のクリープ破断試験により得られた破断時間と対比し、破断時間が母材の破断時間の90%以上であったものをクリープ強度が良好「○」、90%未満であったものをクリープ強度が不十分「×」として評価した。以上の結果を、母材と溶接材料の組合せと併せて表4に示した。
【0077】
【表3】
【0078】
【表4】
【0079】
表4からわかるように、母材および溶接金属ともに本発明で規定する条件を満足し、かつ母材と溶接金属との関係も本発明で規定する条件を満足する代符AJ0〜AJ14の溶接継手は、HAZおよび溶接金属ともに割れは発せず、溶接金属のクリープ破断時間が母材の90%以上で、クリープ強度は良好である。
【0080】
これに対して、母材および溶接金属ともに各元素の含有量は本発明で規定する範囲内ではあるが、母材が本発明で規定する(1)式を満足しない溶接継手、すなわち、(BI/DI)値が1.1を超える代符BJ1〜BJ5の溶接継手は、いずれもHAZで溶接割れが発生し、溶接性が不良である。
【0081】
また、母材および溶接金属ともに、化学組成は本発明で規定する条件を満足するものの、溶接金属のST値と母材のST値との関係が本発明で規定する条件を満足しない代符BJ6〜BJ8の溶接継手は、溶接割れは生じないものの、溶接金属のクリープ破断時間が母材の90%未満で、溶接継手全体としてのクリープ強度は不十分である。
【0082】
【発明の効果】
本発明のNi基耐熱合金からなる溶接継手は溶接部に溶接割れ欠陥がなく、しかも溶接部のクリープ強度が高い。このため、エチレンプラント用分解炉および改質炉の管のように、浸炭および酸化がおきる雰囲気でしかも温度変動が繰り返される使用環境下において優れた特性を発揮する。従って、本発明の溶接継手を有するエチレン分解炉管または改質炉管は、より高温での使用および連続操業時間の延長が可能であり、さらには耐久性の向上により新材との取り替え間隔を長くすることができる。
[0001]
BACKGROUND OF THE INVENTION
  The present invention is a Ni-based heat-resistant alloy having high hot strength and excellent hot workability, weldability and carburization resistance.Consist ofWelded jointAnd even have its welded jointsBased on the welded structure used for cracking furnaces and reforming furnaces for ethylene plantsOn the tubeRelated. The cracking furnace and reforming furnace for ethylene plants are used to produce basic petrochemical products such as ethylene and propylene by cracking or reforming hydrocarbon raw materials such as naphtha, propane, ethane, and gas oil at a high temperature of 800 ° C or higher. It is a furnace.
[0002]
[Prior art]
As for the use temperature of the cracking furnace and reforming furnace for ethylene plants, the tendency to increase in temperature is increasing from the viewpoint of improving the yield of ethylene and the like.
[0003]
Such a cracking furnace and reforming furnace pipe material is required to have high temperature strength and carburization resistance because the inner surface is exposed to a carburizing atmosphere. On the other hand, during the operation, there is a phenomenon that carbon deposits on the inner surface of the tube called coking, and as the amount of precipitation increases, adverse effects on the operation such as an increase in pressure in the tube and a decrease in heating efficiency occur. .
[0004]
Therefore, in actual operation, so-called decoking work is periodically performed to remove carbon deposited by air or water vapor, but the operation stoppage or work man-hours in the meantime becomes a serious problem. Such coking and the problems associated therewith become more serious as the size of the tube becomes a small diameter tube that is advantageous for improving the yield.
[0005]
As a conventional technique for preventing coking, for example, as shown in Japanese Patent Laid-Open No. 2-8336, the alloy surface contains 28% by mass or more of Cr and is strong and stable on the alloy surface.2O3There is a technique that suppresses coking by forming a film and preventing exposure of Fe and Ni, which are catalytic elements that promote carbon deposition, to the surface.
[0006]
On the other hand, in order to improve carburization resistance, it is known that it is effective to increase the Si content in the alloy as disclosed in, for example, Japanese Patent Application Laid-Open No. 57-23050.
[0007]
However, these conventional techniques have the following problems.
[0008]
From the viewpoint of preventing coking, when applying a high Cr alloy as proposed in Japanese Patent Laid-Open No. 2-8336 as a high temperature strength member, it is necessary to increase the amount of Ni in the alloy to make the metal structure austenite. . However, simply making an austenite structure is difficult to apply as a high-temperature strength member alone because the high-temperature strength is lower than that of conventional alloys. Japanese Patent Application Laid-Open No. 2-8336 discloses that a double pipe is used in combination with other high-temperature strength members, but the double pipe has many problems in terms of manufacturing cost and reliability.
[0009]
Further, as disclosed in Japanese Patent Application Laid-Open No. 57-23050, when the Si content in the alloy is increased, there is a problem that the weld cracking sensitivity is increased and the welded structure cannot be put into practical use.
[0010]
On the other hand, as shown in JP-A-4-358037, JP-A-5-239577, JP-A-5-33092 and JP-A-6-207235, the amount of Al in the alloy is increased to the metal surface. Strong and dense Al2O3Alloys that produce a film are significantly improved in carburization resistance and coking resistance as compared to conventional alloys. Further, in such a high Al alloy, by increasing the amount of Ni, the γ 'phase is finely precipitated in the matrix during use at a high temperature, and the creep rupture strength is greatly improved. That is, the alloys shown in these publications are characterized by excellent carburization resistance and coking resistance at high temperatures and high creep strength, and are suitable as cracking furnaces and reforming furnace tubes for ethylene plants.
[0011]
However, the alloy shown in the above publication is not only fully considered in terms of component design with respect to weldability, particularly weld crack resistance, but also sufficient in component design of the weld metal itself that constitutes the welded joint. Careful consideration was not made. Ni-based alloys with a high Al content are not only prone to cracking in the weld heat affected zone (hereinafter also referred to as HAZ) and weld metal during welding, but the weld metal also has a high temperature creep strength compared to the base metal. Tends to be low.
[0012]
Since the weld metal is used as it is in the solidified structure, not in the state of being subjected to hot working or heat treatment like the base material, the high temperature creep strength tends to be low. Therefore, in order to make the material useful for practical use, it is important to incorporate the reduction of cracking susceptibility during welding and the prevention of the decrease in the creep strength of the welded joint in designing the base metal and weld metal components.
[0013]
[Problems to be solved by the invention]
  The object of the present invention is to have excellent carburization resistance and coking resistance in an environment where the cracking furnace and reforming furnace pipes for ethylene plants are placed, that is, in an environment where carburization, oxidation and temperature fluctuation are repeated. Ni-base heat-resistant alloy with excellent weldability and high temperature strengthConsist ofIt is to provide a welded joint.
[0014]
[Means for Solving the Problems]
  The gist of the present invention is the following (1) Ni-base heat-resistant alloyWelded joints consisting ofIt is in. Hereinafter, “%” regarding the content of components means “% by mass”.The
[0016]
  (1) Base material and weld metal are both C: 0.1% or less, Si: 2% or less, Mn: 2% or less, P: 0.025% or less, S: 0.005% or less, N: 0.04% or less, Cr: 10-30%, Al: 2.1-4.5%, and Mo: 2.5-15% or W: 2.5-9% or a total of 2.5-15% of both Mo and W, further Ti: 0-3%, Nb: 0 to 1%, Ta: 0 to 2%, Zr: 0 to 0.2%, Hf: 0 to 0.8%, B: 0 to 0.03%, Mg: 0 to 0.01%, Ca: 0 to 0.01%, Fe : 0-10%, La: 0-0.1%, Ce: 0-0.1%, Nd: 0-0.1%, Y: 0-0.1%, Cu: 0-5%, Co: 0-10%, The restNi And impuritiesAnd satisfying the following equation (1), the ST value of the weld metal obtained by the following equation (2) or (3) is larger than the ST value of the base metal, and the difference is 3 or more: A welded joint made of a certain Ni-base heat-resistant alloy.
[0017]
  (104Si + 1980P + 1980S + 9Al + 15Ti + 11Nb + 1.8W + 11600B)
    ≤ {1.1 (240-20000S-1900P-30Al-10Ti-9W + 17000B)} ... (1),
  When Ti ≦ 4C:ST = Mo + 1.5W + 100Ti (2),
  When Ti> 4C:ST = Mo + 1.5W + 400C (3).
  Here, the element symbols in the formulas (1) to (3) mean the content (mass%) of each element contained in the base material or the weld metal.
[0018]
Among the above components, Ti is preferably 0.005 to 1.0%.
[0019]
  Ni-based alloy of the present inventionWelded joints consisting ofIs particularly suitable for use as tubes in ethylene cracking and reforming furnaces.The
[0020]
DETAILED DESCRIPTION OF THE INVENTION
In order to achieve the above object, the present inventors have a Cr content of 10 to 30%, an Al content of 2.1 to less than 4.5%, and 2.5 to 15% of Mo and 2.5 to 9% of W. As a result of repeated experiments and researches on Ni-based alloys containing one or both of these materials (in the case where both are included, 2.5 to 15% in total), the following was found and the present invention was completed.
[0021]
In HAZ, cracks occur when the boundary of the base metal heated to a high temperature close to the boundary with the weld metal when subjected to the welding heat cycle starts from the part of the base material that has been partially melted. It is caused by propagating to the lower temperature side HAZ where embrittlement has occurred. That is, the weld crack in the HAZ occurs when the welding thermal stress exceeds the fixing force of the grain boundary that has been reduced as described above.
[0022]
Therefore, we quantitatively evaluated the effect of alloying elements on the grain boundary partial melting and the grain boundary embrittlement occurring at a lower temperature than that, and attempted to quantitatively evaluate the resistance.
[0023]
As a result, if the energy release rate A when a crack occurs from a member having a grain boundary melting amount BI under the welding thermal stress R satisfies the following equation (4), the grain boundary is partially It was found that cracking did not occur even when melted.
[0024]
(A = C × R2  × BI) ≦ DI ・ ・ ・ ・ (4)
Here, C is a constant, and DI represents the grain boundary fixing force. BI and DI depend on the chemical composition of the alloy. That is, BI increases as the content of elements that easily cause grain boundary melting (for example, Si, P, Al, Ti, Nb, W, B) increases, and DI tends to cause grain boundary embrittlement ( For example, the larger the content of S, P, Al, Ti, and W), the smaller. Therefore, when the content of elements that easily cause grain boundary embrittlement increases, it becomes difficult to satisfy the equation (4).
[0025]
In theory, the grain boundary melting amount BI is, for each element i, the degree of decrease in liquidus temperature per 1% of the element in the equilibrium diagram, mi, the solid-liquid phase distribution coefficient ki, and the grain boundary segregation. When the amount is bi and the content of each element is Xi, it can be obtained by the following equation (5). As a result of the experiment, it was found that the value can be obtained by the following equation (6).
[0026]
BI = Σ {mi / (bi−1) / ki} × Xi (5)
BI = 104Si + 1980P + 1980S + 9Al + 15Ti + 11Nb + 1.8W + 11600B (6)
In addition, as a result of an experiment for quantifying the influence of each element on the Ni atom bonding strength at the grain boundary by a fracture test at high temperature, the grain boundary fixing force DI is assumed to be a value obtained by the following equation (7). I found it good.
[0027]
DI = 240-20000S-1900P-30Al-10Ti-9W + 17000B (7)
On the other hand, the welding thermal stress R is strictly affected by the welding conditions and the shape and dimensions of the welded joint, particularly the plate thickness and wall thickness. However, in Ni-base heat-resistant alloys mainly composed of TIG welding, the heat input is 20 kJ at most. / Cm, and the influence of the amount of heat input is not significant.
[0028]
However, the influence of the plate thickness on the welding thermal stress is large, and the welding thermal stress R increases rapidly as the plate thickness increases up to a plate thickness of 10 mm. For this reason, a simulated welding experiment was conducted under the most severe complete restraint condition at a plate thickness of 25 mm, which is close to the upper limit of the thickness considered as a welded structure, to investigate the presence or absence of cracks.
[0029]
As a result, if the BI value obtained by the above equation (6) is 1.1 times or less than the DI value obtained by the equation (7), that is, if the components are adjusted so as to satisfy the above equation (1), cracks will occur in the HAZ during welding. It was found that does not occur.
[0030]
Next, on the premise of a base material whose components have been adjusted so as to satisfy the above-mentioned formula (1), the development of a weld metal that has a weld cracking sensitivity that is low in weld cracking and that has the same creep strength as the base material. Tried.
[0031]
The creep strength of the weld metal is lower than that of the base metal. In the base metal, Mo and / or W, which contributes effectively to strengthening by solid solution in the matrix, is more effective in the base metal due to solidification segregation in the weld metal. By not contributing to. Therefore, it is necessary for the weld metal to contain more Mo and / or W than the base metal.
[0032]
However, when the base material has a sufficiently high content of Mo and / or W, it is difficult to contain more Mo and / or W in the weld metal. In order to contain a large amount of Mo and / or W in the weld metal, it is necessary to increase the content of Mo and / or W in the welding material, which reduces the hot workability when processing into the welding material. Because it does.
[0033]
Accordingly, the present inventors have sought a condition in which the creep strength does not become lower than that of the base material even if the weld metal does not contain a larger amount of Mo and / or W than the base material. As a result, it has been found that it is effective to disperse an appropriate amount of TiC at the grain boundaries of the weld metal. More specifically, the ST value obtained by the above-described formula (2) or (3) according to the Ti content contained in the base metal or the weld metal is larger in the weld metal than in the base metal, If the Mo, W, C, and Ti contents of the base metal and the weld metal are adjusted so that the difference is 3 or more, the creep strength of the weld metal, which is a solidified structure, will be almost the same as the creep strength of the base metal. I understood.
[0034]
During multi-layer welding, the weld metal is subjected to the same thermal cycle as HAZ by welding of the next layer, and cracks similar to the HAZ cracks of the base metal are generated. However, it has been found that when the weld metal satisfies the above-described equation (1), no weld cracking occurs as in the case of the base material.
[0035]
  The present inventionWelded joint base metalRegarding the basic composition of the alloy, the present inventors have the viewpoint of satisfying the hot workability that is indispensable as a practical mass-production alloy in addition to the above-mentioned weldability without reducing the carburization resistance and coking resistance at high temperatures. Thus, the following knowledge was obtained as a result of repeated experiments and examinations by melting alloys of various chemical compositions.
[0036]
(a) In alloys containing 1% or more of Al, it is easy to form Al-based nitrides.2O3The protective properties of the Al-based oxide film mainly composed of are lost.
[0037]
(b) However, if Cr containing 10% or more of the effect of increasing the solid solubility of N is contained, if N is reduced, N is sufficiently dissolved and AlN does not precipitate on the alloy surface. Therefore, even if Al is 1% or more, the protection of the oxide film mainly composed of alumina on the alloy surface is not impaired, good carburization resistance and coking resistance can be ensured, and the high temperature strength is improved.
[0038]
(c) When the Al content is suppressed to less than 4.5%, hot workability and weldability are improved, but the hot workability is compared with general Fe-Cr-Ni and Ni-Cr alloys. However, it is not enough when considering mass production. That is, Ni—Al-based intermetallic compounds are precipitated during hot working and the inside of the crystal grains is remarkably strengthened, so that the grain boundary becomes relatively weak. When the inside of the grains is strengthened, a large force must be applied during the processing, so that relatively weak grain boundaries are easily broken and hot workability is lowered. Therefore, in order to improve hot workability, it is necessary to strengthen grain boundaries as well as intragranular strengthening.
[0039]
(d) On the other hand, in Ni-based alloys containing a large amount of Al, the grain boundaries themselves are weakened. One of the main causes of this weakening is S. Therefore, to prevent the grain boundary from weakening, it is extremely important to limit S to 0.005% or less. If it is limited to 0.003% or less, a further improvement effect can be expected.
[0040]
(e) It is important to further reduce N as much as possible. This is because in a Ni-based alloy containing a large amount of Al, as described above, N in steel tends to form Al-based nitrides, and these nitride-based precipitates significantly reduce hot workability.
[0041]
(f) B, Zr, and Hf increase the bonding force of atoms at the grain boundary, and thus are effective in strengthening the grain boundary. Therefore, one or more of these elements are used to prevent deterioration in hot workability. It is good to contain.
[0042]
  Hereinafter, the Ni-base heat-resistant alloy of the present inventionBase metal and weld metal for welded jointsExplain in detail the reason why the chemical composition was determined as above. Less thanThe explanation of each element below is Ni-base heat-resistant alloy unless otherwise specified.Consist ofConfigure welded jointsWith the base materialCommon to weld metal.
[0043]
  Ni-base heat-resistant alloy of the present inventionConsist ofMother constituting the welded jointWith materialsAny weld metal must satisfy the following formula (1).
[0044]
  (104Si + 1980P + 1980S + 9Al + 15Ti + 11Nb + 1.8W + 11600B)
    ≤ {1.1 (240-20000S-1900P-30Al-10Ti-9W + 17000B)} ... (1).
  Here, the element symbol in formula (1) is the alloy of the base metalOrWeld metalinsideIt means the content (% by mass) of each element contained.
[0045]
Satisfying the above equation (1) is an essential condition for preventing weld cracking. The meaning of equation (1) is as follows. As described above, the local fracture stress generated depending on the partial melting amount of the grain boundary in the region immediately below the melting point by welding does not exceed the fracture resistance in the embrittled region of the adjacent grain boundary. This means that a combination of components is selected within the range, and it is possible to prevent the occurrence of weld cracks only when the expression (1) is satisfied.
[0046]
The welded joint of the present invention must satisfy the following conditions.
[0047]
When the respective ST values of the base metal and the weld metal constituting the joint are obtained by the following formula (2) or (3), the ST value of the weld metal is larger than the ST value of the base metal, and the difference is 3 That is all. Only in this case, due to the synergistic effect of solid solution strengthening and strengthening by TiC dispersion, the creep strength of the weld metal, which is a solidified structure, becomes equal to the creep strength of the base metal and balances.
[0048]
  When Ti ≦ 4C:ST = Mo + 1.5W + 100Ti (2),
  When Ti> 4C:ST = Mo + 1.5W + 400C (3).
  However, the above two conditions must be satisfied within the range of the alloy components shown below. This is also clear from the results of Examples described later.
[0049]
C: 0.1% or less
C is an element effective for forming carbides and improving the tensile strength and creep rupture strength necessary as a heat-resistant alloy, so it is desirable that C be contained in an amount of 0.01% or more. However, if its content exceeds 0.1%, not only the ductility and toughness of the alloy are lowered, but also the formation of an alumina film is hindered in a Ni-based alloy containing a large amount of Al. For this reason, C content was made into 0.1% or less. A preferable upper limit is 0.09%, and a more preferable upper limit is 0.07%.
[0050]
Si: 2% or less
Si is an element added as a deoxidizer and contributes to improving oxidation resistance and carburization resistance. However, in Ni-based alloys containing a large amount of Al, the oxidation resistance and carburization resistance are improved. While the effect is relatively small, it has a strong effect of reducing hot workability and weldability. For this reason, it is better to lower the temperature when hot workability is emphasized, especially in manufacturing, but considering that it may be necessary to improve the oxidation resistance and carburization resistance, it is 2% or less. And A preferable upper limit is 1.5%, and a more preferable upper limit is 1%. The effect of improving oxidation resistance and carburization resistance becomes significant at 0.2% or more.
[0051]
Mn: 2% or less
Mn is an element added as a deoxidizer, but it needs to be suppressed to 2% or less because it promotes the formation of a spinel-type oxide film that causes deterioration of coking resistance. A preferred upper limit is 1.5%, and a more preferred upper limit is 1%. The Mn content may be at the impurity level, but it is desirable to contain 0.1% or more in order to ensure the deoxidation effect.
[0052]
S: 0.005% or less
S is an extremely harmful element that segregates at the grain boundaries to weaken the bonding force of the crystal grains and deteriorates the weldability, and the regulation of the upper limit value is extremely important. In particular, it is preferable to reduce the S content as much as possible because grain boundary strengthening is important in a Ni-based alloy containing a large amount of Al. Moreover, in order to improve weldability, it is necessary to make it at least 0.005% or less, so the upper limit was made 0.005%.
[0053]
P: 0.025% or less
P segregates at the grain boundary to weaken the bond strength of the crystal grains, lowers the melting point of the grain boundary, promotes partial melting of the grain boundary at high temperature HAZ (base material portion in contact with the melting boundary), and causes weld cracking. It is a harmful element to be generated. The lower the content, the better. However, if it is 0.025%, there is no particular problem, so the upper limit was made 0.025%.
[0054]
N: 0.04% or less
N is effective and positively used for increasing the strength at high temperatures by solid solution strengthening in general heat resistant steels, but in Ni-based alloys containing a large amount of Al, it precipitates as nitrides such as AlN. In addition, solid solution strengthening cannot be expected, and hot workability and weldability are significantly impaired. Furthermore, the protective film is destroyed starting from the nitride and the carburization resistance is lowered. Therefore, the lower the N content, the better. However, if it is 0.04%, there is no particular problem, and excessive reduction leads to an increase in cost and a decrease in yield, so the upper limit was made 0.04%. A preferred upper limit is 0.03%, and a more preferred upper limit is 0.02%.
[0055]
Cr: 10-30%
Cr is an element effective for improving oxidation resistance and coking resistance, and has an action of uniformly forming an alumina film at the initial stage of formation of the alumina film. Moreover, it forms carbides and contributes to the improvement of creep rupture strength. Further, Cr contributes to improvement of hot workability in the component system defined in the present invention. In order to obtain these effects, at least 10% is necessary. On the other hand, when Cr is excessively contained, mechanical properties such as toughness and workability are inhibited. For this reason, the Cr content is set to 10 to 30%. A preferable range is 12 to 25%, and a more preferable range is 12 to 23%.
[0056]
Al: 2.1% to less than 4.5%
Al is an element that is extremely effective in improving carburization resistance and coking resistance, and further improving high-temperature strength. To obtain the effect, it is necessary to uniformly generate a corundum-type alumina oxide film. Al is the γ ′ phase [Ni3  (Al, Ti) intermetallic compound] to form a precipitation strengthening effect. In order to obtain these effects, at least 2.1% is necessary. On the other hand, if it exceeds 4.5%, the weld cracking sensitivity increases extremely. Therefore, the Al content is set to 2.1 to less than 4.5%. A preferable range is 2.1 to 4%, and a more preferable range is 2.1 to 3.5%.
[0057]
Mo: 2.5-15%, W: 2.5-9%, but if both are included, the total is 2.5-15%
Any of these elements is mainly effective as a solid solution strengthening element, and increases the creep rupture strength by strengthening the base austenite phase. In order to obtain the effect, 2.5% or more is required for each of the total contents of the two kinds. However, if excessively contained, not only intermetallic compounds that cause a reduction in toughness are precipitated, but also carburization resistance and coking resistance are deteriorated. The upper limit should be 15% or less in total of Mo and W. However, since W has a greater decrease in hot workability and weldability due to precipitation of intermetallic compounds than Mo, it is necessary to limit the upper limit of W lower than Mo. Therefore, the content of these elements is 15% or less for Mo and 9% or less for W, and the total content is 15% or less. A preferable range of the total amount is 4 to 13%, and a more preferable range is 6 to 13%.
[0058]
B:
B may not be added. If added, it has the effect of strengthening the grain boundaries, and contributes to a reduction in weld crack sensitivity. Therefore, it can be added when it is desired to obtain this effect, and the effect becomes significant when the content is 0.001% or more. However, if the content of B exceeds 0.03%, the sensitivity to weld cracking is increased. Therefore, the B content when added is preferably 0.001 to 0.03%.
[0059]
Ti:
Ti need not be added. If added, it promotes the precipitation of the γ 'phase and contributes to the improvement of the creep rupture strength, and also precipitates as TiC to strengthen the grain boundary and contribute to the improvement of the creep rupture strength of the weld metal. For this reason, it can add when it wants to acquire these effects, and the effect becomes remarkable when content is 0.005% or more. However, if the content exceeds 3%, the γ 'phase is excessively precipitated and the weldability is remarkably deteriorated. Therefore, the Ti content when added is preferably 0.005 to 3%. A preferred upper limit is 1%.
[0060]
Zr, Hf:
These elements need not be added. If added, any element contributes to the improvement of the creep strength by segregating at the grain boundary and suppressing the grain boundary sliding. For this reason, when it is desired to obtain this effect, one or more kinds can be added, and the effect becomes remarkable at 0.01% or more for any element. However, when Zr exceeds 0.2% and Hf exceeds 0.8%, the creep rupture strength decreases. Therefore, when added, the Zr content is preferably 0.01 to 0.2%, and the Hf content is preferably 0.01 to 0.8%.
[0061]
Mg, Ca:
These elements need not be added. If added, any element mainly fixes S, which is harmful to hot workability, as a sulfide to increase the grain boundary strength and contributes to the improvement of hot workability. For this reason, when it is desired to obtain this effect, one or more kinds can be added, and the effect becomes remarkable when the content of each element is 0.0005% or more. Present in the alloy, conversely reduces hot workability and weldability. Therefore, when added, the content of these elements is preferably 0.0005 to 0.01%. It should be noted that the contents of Mg and Ca when adding these elements are desirably included so that the value obtained by the formula “(1.178Mg + Ca) / S” falls within the range of 0.5 to 3. The effect of improving hot workability is further improved.
[0062]
Fe:
Fe may not be added. If added, it improves creep ductility and contributes to improvement of creep rupture strength, and also contributes to improvement of hot workability and room temperature workability. For this reason, when it is desired to obtain this effect, it can be added, and the effect becomes remarkable when the content is 0.1% or more. However, if it exceeds 10%, the creep rupture strength and hot workability are also reduced. Therefore, the Fe content when added is preferably 0.1 to 10%.
[0063]
  Nb, Ta:
  These elements need not be added. When added, any element contributes to the improvement of creep rupture strength by forming a carbide in addition to strengthening the matrix by dissolving in the austenite phase. For this reason, in order to obtain this effect, one or more kinds can be added, and the effect becomes remarkable when the content of each element is 0.01% or more.Nb Is 1If the amount exceeds 20%, and Ta exceeds 2%, toughness is reduced. Therefore, when addingNb ofThe content is preferably 0.01 to 1%, and the content of Ta is preferably 0.01 to 2%. In addition,Nb ofThe preferable upper limit is 0.8%, and the preferable upper limit of Ta is 1.8%.is there.
[0064]
La, Ce, Nd, Y:
These elements need not be added. If added, any element has the effect of mainly preventing the peeling of the alumina film under thermal cycling conditions and improving the carburization resistance and coking resistance even when used in an environment where the temperature varies. Therefore, when it is desired to obtain the effect, one or more kinds can be added, and the effect becomes remarkable when the content of each element is 0.002% or more. However, if any element exceeds 0.1%, not only the effect of preventing the alumina film from being peeled off but also the workability deteriorates. Therefore, the content of these elements when added is preferably 0.002 to 0.1% for all elements.
[0065]
Cu, Co:
These elements need not be added. If added, each element mainly contributes to stabilization of the austenite phase, and Co also contributes to improvement of creep strength by solid solution strengthening. For this reason, when it is desired to obtain these effects, one or more kinds can be added, and the effect becomes remarkable when the content of each element is 0.01% or more. However, when Cu exceeds 5% and Co exceeds 10%, toughness and workability are impaired. Therefore, when added, the Cu content is preferably 0.01 to 5%, and the Co content is preferably 0.01 to 10%. In addition, the upper limit with preferable Cu content is 3%, and a more preferable upper limit is 1.5%, the preferable upper limit of Co content is 8%, and a more preferable upper limit is 5%.
[0066]
  Of the present inventionUsed as a base material for welded jointsThe Ni-base heat-resistant alloy is obtained by casting after melting in a normal melting and refining process, and can be used as it is. Usually, after casting, it goes through various processing steps such as forging, hot working and cold working.UseYes. In powder metallurgy,ManufacturingMay be. The heat treatment promotes the homogenization of the structure, and the present inventionWelded jointsContributes to improved performance. As the heat treatment, a homogenization treatment at 1100 to 1300 ° C. is preferable, but it can be used as cast or processed.
[0067]
  In addition, the present inventionUsed as a base material for welded joints Ni Base heat resistant alloyIs processed into a wire and used as a co-welded welding materialButThis can be used to obtain a welded joint by TIG welding or the like. The weld metal in that case is made into the said composition from a viewpoint of use performance and a weld crack prevention. Within this composition range, it is necessary that the ST value of the weld metal obtained by the above formula (2) or (3) is larger than the ST value of the base metal and the difference is 3 or more. Thereby, a weld metal having a creep strength equivalent to that of the base material is obtained.
[0068]
The weld metal is formed by melting and mixing the welding material and a part of the base material. Therefore, the composition of the weld metal can be adjusted by selecting the composition of the weld material in consideration of the base material composition and the dilution rate.
[0069]
【Example】
Twenty kinds of base materials having chemical compositions shown in Table 1 and nine kinds of welding materials having chemical compositions shown in Table 2 were prepared. Both the base material and the welding material were melted using a high-frequency vacuum melting furnace with a capacity of 50 kg, and the obtained ingot was hot forged into a plate material with a plate thickness of 25 mm and subjected to solution heat treatment at 1250 ° C. The applied material was used as a base material. Further, the above ingot was processed into a wire with an outer diameter of 2 mm through hot forging, hot rolling, and cold drawing steps to obtain a welding material.
[0070]
[Table 1]
[0071]
[Table 2]
[0072]
Using the prepared base material and welding material, a welded joint production test was performed according to the following procedure.
[0073]
From each base material, two test pieces having a width of 100 mm and a length of 200 mm and V-groove processing of 20 ° half-width on one of the long sides were collected. The two test pieces were face-to-face with V-groove processing, placed on a steel plate with a thickness of 50 mm, a width of 150 mm, and a length of 250 mm, and all four sides were covered by arc welding (3 Pass) and fully restrained. Next, the welded V groove portion was multilayer welded by the TIG welding method. The welding conditions were a welding current of 130 A, a welding voltage of 12 V, and a welding speed of 15 cm / min. Various combinations of the prepared base metal and welding material were used to prepare 23 types of welded joints having weld metals with chemical compositions shown in Table 3.
[0074]
Weldability (weld crack resistance) is evaluated from the obtained welded joints in the direction in which the longitudinal direction is perpendicular to the weld line and the center of the weld metal in the width direction is located at the center in the width direction. Take 10 mm and 200 mm long side bend specimens, bend 180 degrees with a bend radius of 20 mm, and look at the surface of the bend with an enlarged field of view of 50 times, and crack in HAZ and weld metal No cracking was observed on all five test pieces, and the weld cracking resistance was good for those with no cracking. The property was evaluated as “x”.
[0075]
For welded joints where no cracks were observed, the longitudinal direction was perpendicular to the weld line, and there was a parallel part with an outer diameter of 6 mm and a length of 30 mm at the center of the longitudinal direction. A creep rupture test piece with the weld metal located in the center was collected and subjected to a creep rupture test under the conditions of a temperature of 1150 ° C. and a load stress of 7 MPa to investigate the rupture time of the weld metal.
[0076]
The creep strength of weld metal was evaluated by taking a rupture time of 90% or more of the rupture time of the base metal in comparison with the rupture time obtained by the creep rupture test taken from the base metal under the same conditions as above. The creep strength was evaluated as “Good”, and the creep strength of less than 90% was evaluated as “Poor”. The above results are shown in Table 4 together with combinations of base materials and welding materials.
[0077]
[Table 3]
[0078]
[Table 4]
[0079]
As can be seen from Table 4, both the base metal and the weld metal satisfy the conditions specified in the present invention, and the relationship between the base metal and the weld metal also satisfies the conditions specified in the present invention. The HAZ and the weld metal do not crack, the creep rupture time of the weld metal is 90% or more of the base metal, and the creep strength is good.
[0080]
On the other hand, the content of each element in both the base metal and the weld metal is within the range specified in the present invention, but the base metal does not satisfy the formula (1) defined in the present invention, that is, (BI / DI) All of the welded joints of the symbol BJ1 to BJ5 exceeding 1.1 have weld cracks in the HAZ and have poor weldability.
[0081]
In addition, the chemical composition of both the base metal and the weld metal satisfies the conditions specified in the present invention, but the relationship between the ST value of the weld metal and the ST value of the base metal does not satisfy the conditions specified in the present invention BJ6 Although the weld joint of ˜BJ8 does not cause weld cracking, the creep rupture time of the weld metal is less than 90% of the base metal, and the creep strength of the weld joint as a whole is insufficient.
[0082]
【The invention's effect】
  Ni-base heat-resistant alloy of the present inventionMade ofThe joint has no weld crack defect in the welded portion, and the creep strength of the welded portion is high. For this reason, it exhibits excellent characteristics in an environment where carburization and oxidation occur, such as a cracking furnace for an ethylene plant and a reforming furnace, and in an environment where temperature fluctuations are repeated. Therefore, the present inventionEthylene cracking furnace tube or reforming furnace tube with welded jointCan be used at a higher temperature and the continuous operation time can be extended, and the interval between replacement with a new material can be extended by improving the durability.

Claims (3)

母材および溶接金属がいずれも、質量%で、C:0.1%以下、Si:2%以下、Mn:2%以下、P:0.025%以下、S:0.005%以下、N:0.04%以下、Cr:10〜30%、Al:2.1〜4.5%未満、並びにMo:2.5〜15%もしくはW:2.5〜9%またはMoとWを合計で2.5〜15%含み、さらに、Ti:0〜3%、Nb:0〜1%、Ta:0〜2%、Zr:0〜0.2%、Hf:0〜0.8%、B:0〜0.03%、Mg:0〜0.01%、Ca:0〜0.01%、Fe:0〜10%、La:0〜0.1%、Ce:0〜0.1%、Nd:0〜0.1%、Y:0〜0.1%、Cu:0〜5%、Co:0〜10%を含み、残部がNi および不純物よりなり、かつ下記の(1)式を満足するとともに、下記の(2)式または(3)式で求められる溶接金属のST値が母材のST値よりも大きく、その差が3以上であるNi基耐熱合金からなる溶接継手。
(104Si+1980P+1980S+9Al+15Ti+11Nb+1.8W+11600B)
≦{1.1(240−20000S−1900P−30Al−10Ti−9W+17000B)}・・・(1)
Ti≦4Cの場合:ST=Mo+1.5W+100Ti・・・(2)
Ti>4Cの場合:ST=Mo+1.5W+400C・・・(3)
ここで、(1)〜(3)式中の元素記号は、母材または溶接金属中に含まれる各元素の含有率(質量%)を意味する。
Both the base metal and the weld metal are in mass%, C: 0.1% or less, Si: 2% or less, Mn: 2% or less, P: 0.025% or less, S: 0.005% or less, N: 0.04% or less, Cr : 10 to 30%, Al: 2.1 to less than 4.5%, and Mo: 2.5 to 15% or W: 2.5 to 9% or Mo and W in total 2.5 to 15%, further Ti: 0 to 3%, Nb: 0 to 1%, Ta: 0 to 2%, Zr: 0 to 0.2%, Hf: 0 to 0.8%, B: 0 to 0.03%, Mg: 0 to 0.01%, Ca: 0 to 0.01%, Fe : 0-10%, La: 0-0.1%, Ce: 0-0.1%, Nd: 0-0.1%, Y: 0-0.1%, Cu: 0-5%, Co: 0-10%, The balance is Ni and impurities , and satisfies the following formula (1), and the ST value of the weld metal obtained by the following formula (2) or (3) is larger than the ST value of the base metal, Welded joint made of Ni-based heat-resistant alloy with a difference of 3 or more.
(104Si + 1980P + 1980S + 9Al + 15Ti + 11Nb + 1.8W + 11600B)
≤ {1.1 (240-20000S-1900P-30Al-10Ti-9W + 17000B)} ... (1)
When Ti ≦ 4C: ST = Mo + 1.5W + 100Ti (2)
When Ti> 4C: ST = Mo + 1.5W + 400C (3)
Here, the element symbols in the formulas (1) to (3) mean the content (mass%) of each element contained in the base material or the weld metal.
Tiが0.005〜1.0%である請求項1に記載の溶接継手。  The welded joint according to claim 1, wherein Ti is 0.005 to 1.0%. 請求項1または2に記載の溶接継手を有するエチレン分解炉管または改質炉管。  An ethylene decomposition furnace tube or a reforming furnace tube having the welded joint according to claim 1 or 2.
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